1. Field of the Invention
The invention relates to elements faced with superhard material, and particularly to preform cutting elements comprising a facing table of superhard material having a front face, a peripheral surface, and a rear surface bonded to a substrate of material which is less hard than the superhard material.
2. Description of Related Art
Preform elements of this kind are often used as cutting elements on rotary drag-type drill bits, and the present invention will be particularly described in relation to such use. However, the invention is not restricted to cutting elements for this particular use, and may relate to preform cutting elements for other purposes. For example, elements faced with superhard material, of the kind referred to, may also be employed in workpiece-shaping tools.
Preform elements used as cutting elements in rotary drill bits usually have a facing table of polycrystalline diamond, although other superhard materials are available, such as cubic boron nitride and amorphous diamond-like carbon (ADLC). The substrate of less hard material is often formed from cemented tungsten carbide, and the facing table and substrate are bonded together during formation of the element in a high pressure, high temperature forming press. This forming process is well known and will not be described in detail.
Each preform cutting element may be mounted on a carrier in the form of a generally cylindrical stud or post received in a socket in the body of the drill bit. The carrier is often formed from cemented tungsten carbide, the surface of the substrate being brazed to a surface on the carrier, for example by a process known as "LS bonding". Alternatively, the substrate itself may be of sufficient thickness as to provide, in effect, a cylindrical stud which is sufficiently long to be directly received in a socket in the bit body, without being brazed to a carrier. The bit body itself may be machined from metal, usually steel, or may be moulded using a powder metallurgy process.
Such cutting elements are subjected to extremes of temperature during formation and mounting on the bit body, and are also subjected to high temperatures and heavy loads when the drill is in use down a borehole. It is found that as a result of such conditions spalling and delamination of the superhard facing table can occur, that is to say the separation and loss of the diamond or other superhard material over the cutting surface of the table.
This may also occur in preform elements used for other purposes, and particularly where the elements are subjected to repetitive percussive loads, as in tappets and cam mechanisms.
Commonly, in preform elements of the above type, the interface between the superhard table and the substrate has usually been flat and planar. However, particularly in cutting elements for drill bits, attempts have been made to improve the bond between the superhard facing table and the substrate by configuring the rear face of the facing table and the front face of the substrate so as to provide a degree of mechanical interlocking between them.
It is also desirable, in preform elements of the above type, to provide increased thickness of the superhard facing table at its periphery to provide additional strength in that region which, in a cutting element for a rotary drill bit, provides the cutting edge of the element. Normally this has been achieved by forming a continuous peripheral ring of greater thickness and constant depth around the outer periphery of the rear surface of the facing table. In known arrangements it is often necessary or desirable for the projections on the rear surface of the facing table to be of greater depth than the peripheral ring, in order to provide adequate mechanical interlocking with the substrate.
Various configurations of the interface between the superhard facing table and substrate in a preform element are described in British Patent Applications Nos. 2283773, 9512173.7, 9512174.5, 9512177.8 and 9512175.2.
While some of these existing designs can provide advantages, it may sometimes be found that the projections on the rear surface of the facing table require to project into the substrate to a considerable depth in order to provide an adequate locking function. For example, in some types of preform element the thickness of the substrate may be less than 3mm, and if the projections on the facing table are extended into the substrate to provide adequate interlocking between the facing table and substrate they may have the undesirable effect of weakening the structure of the substrate. This difficulty might, of course, be overcome by increasing the thickness of the substrate, but not only may thicker preforms be more difficult and costly to manufacture, but difficulties are also likely to arise due to existing manufacturing processes and designs of drill bit already being geared to preforms of the standard thickness.
The present invention sets out to provide a design of preform element where the configuration of the projections on the rear surface of the facing table is such that they provide good interlocking between the facing table and substrate with comparatively small depth of the projections to the rear of the facing table.
Preform elements of the kind to which the present invention relates are usually manufactured by first pre-forming a shaped solid substrate from suitable material, such as tungsten carbide, and then applying to one surface of the substrate a layer of diamond or other superhard particles. The superhard layer then automatically conforms to the shape of the substrate surface, the particles filling any recesses which have been pre-formed in that surface. When the substrate and superhard layer are bonded together in the high pressure, high temperature forming press, the diamond particles bond together and to the substrate, and the rear surface of the superhard facing table becomes integrally formed with projections of superhard material which extend into the recesses in the substrate.
As previously explained, certain characteristics of the finished preform element may depend on the shape and configuration of these superhard projections. However, since the projections are usually, in practice, moulded according to the shape of the pre-formed substrate, it is convenient to define a desired configuration of superhard projections in terms of the shape of the substrate which is required to produce them, and the present invention will therefore be defined in such terms.